The preparation of polyurethane resins of either foamed or compact type by the reaction of organic polyisocyanates with polyether-polyols in the presence of suitable catalysts and additives is well-known from relevant technical literature.
In European patent application No. 10,850, a class of liquid polyisocyanates is disclosed, which is used, in particular, in the preparation of cold-aged flexible polyurethane resins, characterized by minimal surface defects.
Polyisocyanate compositions disclosed in this patent application are those which comprise:
A. from 90 to 50% by weight of the reaction product of diphenyl-methane-diisocyanate and a polyoxyalkylene polyol having an average functionality of from 2 to 3, and containing from 50 to 100% by weight of a polyoxypropylene diol or triol having an equivalent weight within the range of from 750 to 3000 and from 0 to 50% by weight of a polyoxyethylene-diol or triol having a molecular weight within the range of from 750 to 3000 and wherein the content of free NCO radicals, in said reaction product, is within the range of from 8 to 26% by weight; and
B. from 10 to 50% by weight of a composition containing from 30 to 65% by weight of diphenyl-methane-diisocyanate and from 70 to 35% by weight of polymethylene-polyphenyl-polyisocyanates having a functionality higher than 2.
The above described polyisocyanate compositions are used as the isocyanate component in the production of polyurethanes by being reacted with organic polyols under conventional conditions or, more particularly, in the production of polyurethane foams in case during polycondensation suitable foaming agents are added to the reaction mixture.
However, the foams obtained from these compositions display very closed cell structures, so that further treatments--of the type of beating or of mangling--are required to break the cells of the foam and favor the escaping of the foaming gases remaining inside the interior of said cells.